This invention relates to a cleaning apparatus employing a doctor blade for use in an electrophotographic copier.
In an ordinary dry type electrophotographic copier, a series of copying process steps comprise: forming a latent image of an original on a rotating photosensitive drum coated with a photoconductive insulating substance on its surface; rendering visible the latent image by a developing step using a developing agent comprising a toner, made of a resin powder colored by a coloring agent such as carbon black, and a carrier. In a transcribing step, the visible toner image is transferred to an image supporting body such as paper. A portion of the toner inevitably remains on the surface of the photosensitive drum. This residual toner must be removed by a cleaning apparatus before the copier proceeds to a subsequent copying operation. The photosensitive drum from which the residual toner has been removed is then irradiated by a lamp thereby discharging any residual potential from the surface of the drum.
As for the cleaning apparatus for removing the residual toner, a doctor blade cleaning device made of a polyurethane sheet has been widely used in place of a device such as a rotating brush made of rayon wool or the like.
The doctor blade cleaning apparatus is advantageous in that the size thereof is smaller than the device having a rotating brush. Its electrical power consumption is low, and no vacuum mechanism is required. However, since the doctor blade is brought into contact with the surface of the photosensitive drum for scraping off the residual toner, the doctor blade is susceptible to the intrusion of foreign matter into the nip between the doctor blade edge and the photosensitive drum. Furthermore, there has been a tendency for the removed toner to not entirely drop off the doctor blade but to be partly retained at the blade edge. These phenomena not only reduce the cleaning effect of the doctor blade cleaning apparatus, but also tend to damage the photosensitive substance of the drum. The foreign matter consists mostly of carrier particles and paper dust, which deteriorate the cleaning ability of the apparatus and degrade the copies obtained from the copier. Furthermore, since the doctor blade in the doctor blade cleaning apparatus is brought into contact with the photosensitive drum under a predetermined pressure, a lubricating agent is normally applied for reducing the frictional resistance between the blade edge and the drum.
Lubricating agents such as zinc stearate (Zn(C.sub.17 H.sub.35 COO).sub.2), polyvinylidene fluoride (--CF.sub.2 --CH.sub.2 --) and the like have been widely used. They are applied by the developing agent by being mixed with the toner. However, it has been found that toner containing such lubricating agents accelerates the deterioration of the developing agent. The reason for this is that the lubricating agent impacts and coats the carrier thereby preventing the normal triboelectric charging by friction between the toner and carrier. For this reason, mixing the lubricating agent in the toner is not desirable. Furthermore, since the above described lubricating agents can comprise white powder of a comparable grain size to that of the toner, it has been found that white dots appear in the copies obtained by the copying process. These dots are not desirable and they reduce the quality of the finished copies.